A spokesman told reNews two divers were working underwater guiding the placement of six-tonne concrete mats over subsea cables when one of them was trapped under a mat.

"Investigations are ongoing by the police and authorities but it seems that it was a very tragic sequence of events - many things coming together - that led to the accident," he said.

The spokesman clarified earlier reports that the rescue had been "unsuccessful" by stressing that procedures were followed promptly at the time, including the immediate attendance of a search diver to aid the casualty's co-diver. Lifting straps were placed in position, the concrete mat was lifted quickly and the casualty was retrieved immediately to the dive support vessel, he said.

Unfortunately, the doctor on board could not resuscitate the victim. The spokesman said it appears he was killed instantly by the mat.

"Faced with this painful loss we are all deeply saddened," said project manager Wilfried Hube. "The thoughts of the entire Riffgat team are with the relatives of the victim and his fellow divers."

EWE said the police and relevant authorities had spent the weekend at the site gathering information and evidence to try to piece together what happened. Further information will be available in the next few days, the spokesman added.

Diving operations have been suspended at Riffgat and colleagues and fellow divers are being given support from professionals, the spokesman said. "The divers are now taking a few days off. When they say they are able to work again, they will resume," the spokesman said.

Separate operations to install the farm's turbines are nearing completion with the last Siemens machine expected to be installed either today or tomorrow.